PSI - Issue 13

A.A. Alabi et al. / Procedia Structural Integrity 13 (2018) 877–885 Alabi et al / Structural Integrity Procedia 00 (2018) 000–000

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Keywords: Ductile-to-brittle transition curve; high strength structural steel; loading rate; Master Curve; transition temperature; yield-to-tensile ratio.

1. Introduction Often, an understanding of fracture behaviour of steel during experimental testing at different loading regimes helps to prevent some of the potential catastrophic accidents during in-service conditions. It is assumed that a single fracture toughness value (critical value) controls the fracture behaviour of a material [ Wallin (2011) ]. This value describes the crack initiation and subsequent propagation behaviour of the material (the driving force and the material resistance). The driving force is the combination of material flaw size and loading conditions, while the material resistance is the ability of the material to resist propagation of these flaws or cracks [ Wallin (2011) ] . The strain-hardening exponent ( n ) influences fracture behaviour [ Bannister and Trail (1996) ]. It has been demonstrated that crack opening is enhanced by a high yield to-tensile (Y/T) strength ratio and, hence low strain-hardening capacity [ Bannister (1999) ]. The effect of loading rates on the difference in the dynamic and quasi-static fracture toughness values for a steel material relies mainly on the material’s deformation properties, with the brittle cleavage fracture process remaining the same, but the difference in fracture toughness predicted to be material dependent [ Wallin (2011) ]. In some cases, ductile fracture is considered to have a positive rate dependence with an enhancement of dynamic ductile fracture toughness, however this factor is considered negligible when performing structural analysis [ Walters and Przydatek (2014) ]. Brittle fracture toughness of ferritic steels generally reduces in value with increasing loading rate (a negative rate dependence) [ Wiesner and MacGillivray (1999), Wallin (2011) ]. The mechanism of brittle fracture is mainly controlled by the stress state in front of the crack, and less affected by adiabatic heating because its initiation is in the region of high stresses where the plastic strains are relatively small, further away from the crack tip [ Wallin (2011) ]. This implies that the yield strength and strain-hardening properties of a steel material have an effect on the brittle fracture toughness resistance. A significant impact may be experienced on the ductile-to-brittle-transition curve where a brittle fracture toughness may drop up to 80% from the measured toughness at quasi-static conditions [ Wallin (2011) ]. Thus, the effect of loading rate must be accounted for in the estimation of brittle fracture toughness resistance of high strength structural steel (HSSS) with high Y/T ratio above 0.90. In this paper, a short survey and outlook of the effect of loading rate on fracture toughness of ferritic steel is presented alongside the structural implication of high Y/T ratio in ferritic steel. Finally, test data at quasi-static and elevated loading rates show how fracture toughness behaviour in terms of the transition temperature of S690QL and S960QL with Y/T ratio above 0.90 changes due to elevated loading rate.

Nomenclature a 0 initial crack length (mm) n Strain-hardening exponent σ y

Yield strength (MPa or N/mm 2 ) specimen thickness (mm) ratio of yield to tensile strength

B

Y/T HAZ

Heat affected zone

1T

specimen size at 1 in thickness, i.e. B = 25.4 mm

E Modulus of elasticity (GPa) K Stress intensity factor (MPa√m) � Stress intensity factor loading rate (MPa√m/s) K JC elastic-plastic equivalent stress intensity factor derived from J-integral at onset of cleavage fracture � Strain rate (s -1 ) a 0 /W ratio of the initial crack growth to the width SENB single edge notched bend W specimen width, measured in the direction of the notch (mm) Δ T Temperature shift (°C) T 0 quasi-static reference transition temperature (°C) σ y T ₀ Yield strength estimated at T 0 (MPa)